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Motorsport Wheel Alignment: Toe

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Toe

05.54

00:00 - Toe is another key component that we'll be making changes to when adjusting our wheel alignment and in order to understand and visualise what toe is, we need to view the car from above.
00:11 While you may assume that when the steering wheel is straight, all four wheels are simply pointing straight ahead, this is actually seldom the case.
00:19 In reality the wheels will either provide some amount of toe in, which is when the front of the wheel is closer to the centreline of the car than the rear or toe out which is when the front of the wheel is further away from the centreline of the car compared to the rear.
00:34 Toe specifications can be expressed as an angle in degrees from the vehicle centreline, however for our purposes they're normally stated as the difference between the measurement at the front of the wheel and the back of the wheel to the centreline of the car in either millimetres or inches.
00:50 We do need to be a little careful however when comparing toe measurements as toe can be expressed for an individual wheel, such as for example one millimetre of toe out or it can represent the total toe for an axle.
01:04 If for example both front wheels had one millimetre of toe out, then the total toe on this axle would be referred to as two millimetres total toe.
01:14 The difference between a toe measurement in measurement and an angle will also vary depending on the rim diameter.
01:20 For example a toe in measurement of one millimetre when applied to an 18 inch diameter wheel will result in less angle compared to the same one millimetre toe in setting on a 15 inch diameter wheel.
01:32 The numbers that we will reference for this course are generally going to apply to a wheel rim in the 17 to 18 inch range which is a popular choice.
01:42 Toe has a dramatic effect on the stability of our car, particularly at high speed.
01:47 It can also affect how sharply the car will turn into a corner, how stable the car is under heavy braking, how it responds through the corner and it can have a big impact on our tyre life.
02:00 In a factory car, we're likely to find that that the stock alignment settings call for a small amount of toe in at the front and the rear of the car.
02:08 This toe setting means that each wheel is essentially trying to drive ever so slightly towards the centreline of the car and this helps make the car feel stable and predictable.
02:19 Since factory alignment specs will only call for a relatively small amount of toe, this also limits tyre scrub and helps prolong tyre life.
02:27 We'll be talking about tyre scrub shortly.
02:30 In a race application on the other hand, we're more likely to run toe out on the front of the car as this helps overcome understeer and promotes sharp turn in at the corner entry.
02:40 At the rear we're still likely to run toe in as this helps make the car stable under hard braking and acceleration.
02:47 The amount of toe is going to depend on a number of aspects and it's important to consider these things and how they'll affect your choices.
02:55 First of all we need to understand the effect of tyre scrub.
02:59 When our toe is zero and the tyre is rolling in the same direction that the car is travelling, it will roll evenly over the racetrack and there is no tyre scrub occurring.
03:09 As we adjust our toe setting however, the tyre is now forced to travel slightly sideways or scrub across the track surface as the wheel turns.
03:17 This generates heat in the tyre due to friction between the tyre and the track and this gets worse as we increase the amount of toe or the speed that the car is travelling at.
03:28 With very aggressive toe settings this can significantly reduce the tyre life expectancy and this becomes most noticeable on racetracks with long straights where speeds tend to be higher and hence more heat is generated.
03:41 Excessive toe can also hamper top speed due to the high level of tyre scrub being generated.
03:47 For this reason, we can generally run more toe on tight low speed tracks than what we would use on a faster racetrack with higher speed corners and longer straights.
03:57 We also need to consider the effect that the suspension bushes have on the toe settings.
04:02 This is easy to overlook however in stock form, all cars use rubber bushes between the suspension components and the chassis to help isolate noise and remove vibraton and harshness from the chassis.
04:14 These bushes however do move around and deflect under high load which means that the actual toe settings out on the racetrack may not be the same as the static settings that you apply on a wheel alignment machine.
04:26 For an example of this, let's consider the rear suspension where we have a conventional rubber trailing arm bush and we've set the toe to zero.
04:34 This may be fine during acceleration however when we brake hard, the braking forces generated by the rear wheels will tend to deform the trailing arm bush and this can allow the wheels to move into toe out.
04:46 This can in turn make the car become very unpredictable and skittish when braking as the rear of the car may feel loose and unstable depending on which rear wheel is most heavily loaded.
04:58 At the rear of the car, we're almost always going to use a small amount of toe in to prevent this instability we've just talked about.
05:05 There are a few limited uses where toe out may make sense, such as a front wheel drive car that's being used for low speed autocross events where toe out at the rear can help the car to rotate into a corner, overcoming the natural tendancy to understeer.
05:21 This needs to be treated with a lot of caution though as it can make the car unpredictable and very dangerous at higher speeds.
05:28 While too much toe will affect tyre life as we've already discussed, it can also make the car unpredictable when it negotiates a bump that only affects one wheel or a split traction surface such as where one wheel runs through standing water on the racetrack.